1. Field of the Invention
This invention relates generally to photographic film printers, and more particularly to high speed photographic film printers suitable for reproducing commercial quality movie film in large quantities.
2. Description of the Prior Art
Photographic film printers are known. Such photographic film printers generally employ a pair of printing heads, one for printing visual information and the other for printing the sound track. Each of the printing heads contains a sprocket for advancing a sound track negative, a picture negative and unexposed raw stock across the printing heads where the raw stock is exposed. The sprockets within the printing heads are generally driven by an electric motor coupled to the sprockets by a gear mechanism. Intermediate sprockets, that are usually driven by the same electric motor that drives the sprockets within the printing heads, are used to apply a differential tension across the sprockets within the printing heads in order to maintain the sprocket holes of the film in engagement with the sprockets of the printing heads. Such a differential tension maintains the film in contact with the printing heads and minimizes the relative motion of the negatives and the raw stock. Pay out and take up reels are provided for the raw stock and for the sound track and picture negatives. Such pay out and take up reels are generally driven by electric motors coupled to the spindles supporting the pay out and take up reels by means of clutches or similar devices designed to apply a predetermined torque to the various spindles. Sometimes, other auxiliary sprockets were also needed to guide the film between the pay out and take up reels and across the printing heads.
While such prior art printers provide a way to duplicate photographic film, the intermediate sprockets required to maintain the differential tension across the printing heads and the auxiliary sprockets used for guiding the film are costly and tend to wear out the sprocket holes in the negatives prematurely. The overall system has a relatively high rotating mass which makes it difficult to control the film accurately. The control of the various motors during the starting and stopping of the printer is relatively imprecise, thus resulting in a large amount of wasted film during the starting and stopping operation, and an increase in the probability of film break during such a starting or stopping operation. In the event of such a break, the relatively high rotating mass results in a large amount of film spillage. The high rotating mass and clutches result in high power comsumption, and light sources within the printing heads also generate a large amount of heat that is destructive to film. Finally, the prior art printers are complex and difficult to repair.